We plan to continue our studies on chromatin stucture by addressing the following three (3) specific aims: 1. To determine the orientation of histone H1 molecules along polynucleosome chains. That is, to determine if histone H1 is oriented in a head-to-head or a head-to-tail (or both) fashion. For this purpose, histone H1 homodimers will be isolated after chemical cross-linking of chromatin. Positions of cross-linking will be mapped after the cleavage of specific peptide bonds and the delineation of the resulting products. 2. To determine the contact points between HMG-17 and the histone octamer. Depleted nucleosomes will be reassociated with HMG-17 and cross-linked with various chemicals. Protein dimers composed of HMG-17 and core histones will be identified and isolated. The positions of cross-linking will be deduced by employing peptide mapping techniques. 3. To use antibodies as accessibility probes of nucleosome structure. Monoclonal antibodies against HMG-17 and histone H2A.Z will be isolated. The positions within the primary structures of HMG-17 and H2A.Z that react with these antibodies will then be determined (by staining blots of peptides). The extent of binding of these antibodies to nucleosomes will then allow judgment of whether specific regions of protein molecules are buried or accessible.